1. Field of the Invention
The present invention relates to laser systems. More specifically, the present invention relates to systems and methods for generating and amplifying laser energy.
2. Description of the Related Art
High energy and high power laser systems generate optical beams for applications such as laser weapons, industrial lasers for materials processing, and long range lidar/ladar systems. (Energy and power are related, but fundamentally different, physical parameters. The challenges in scaling lasers to higher energy and power are often similar, but not always so. The present invention can be adapted to either energy or power scaling. Throughout the remainder of this discussion, the term power will be used with the understanding that this invention can also be used to increase energy.) One of the most challenging problems for scaling laser output power has proven to be beam quality reduction, which arises from the increased cross-sectional area of the active medium, as well as from thermal effects, optical damage, and nonlinear processes that are initiated in the laser materials at high optical intensities.
Solid-state lasers (SSLs) have many packaging advantages when compared to liquid or gas lasers. Diode pumped SSLs appear to be among the most power-efficient laser designs. In general, there are two alternative routes to diode-pumped SSL systems today. Bulk active elements for SSL applications are shaped as rods or slabs, or, recently, as planar waveguides (PWGs) that allow for a large beam cross-section to eliminate optical damage and to provide a more favorable thermal geometry. Bulk elements are usually made of a heat conducting crystalline material to facilitate cooling. Large size crystals, however, suffer from poor optical quality, and are expensive to manufacture. This factor limits the power scalability of crystalline bulk systems.
Active elements of SSLs can also be made from glass. Glass is inexpensive, clear, isotropic and a low-absorbing material. Cooling bulk glass elements is complicated, however, because of the low thermal conductivity of glass.
The optical fiber configuration solves this thermal management problem: the large surface-area-to-volume ratio can maintain a low core temperature even at high lateral gradient. Fiber lasers have proven to be more power-effective SSL devices than bulk rods or slabs. They include a small active core that provides for excellent spatial overlap between absorbed pump power and the signal beam and ensures heavily saturated operation, both of which make fiber lasers highly efficient. Moreover, fibers can be designed to minimize power lost due to fluorescence and amplified spontaneous emission (ASE). In addition, fiber designs are compact, flexible, and lightweight. Exceptional power efficiency and significant packaging benefits have positioned fiber lasers as one of the most promising candidates for high power laser applications, including airborne high-energy weapons and lidar/ladar sensors.
On the other hand, the small cross-sectional area of fibers, which is advantageous for beam quality, heat dissipation, and gain saturation, becomes a real problem if one wishes to scale the output power to higher levels. Optical damage and nonlinear optical effects both arise at high light intensity. So, when attempting to increase laser output while maintaining high beam quality (BQ), the dilemma of choosing between bulk crystal or glass active elements and fiber gain elements has not yet been resolved.
Considering the specific case of fiber-based lasers and amplifiers, the applicability of traditional active fibers for building high-energy laser and long distance lidar/ladar sources is limited. Single transverse mode operation is typically necessary for high intensity on target, for accurate pointing, and for beam control, but such operation usually requires that the core diameter be small. However, if the core is too small, optical damage can preclude reliable operation. In modern silica glass fibers, reliable operation requires intensities to be maintained below 2 W/μm2 (200 MW/cm2) for CW (continuous wave) applications and about 2 GW/cm2 for nanosecond-range pulses. Nonlinear phase modulation and various types of stimulated scattering occur for common fibers at power levels of several hundred watts, resulting in spectral broadening, locking longitudinal modes, and degraded coherence. Thermal damage of pump combiners represents another challenge.
Large mode area (LMA) fibers are commonly used for high power fiber lasers to overcome damage and nonlinearity problems. However, the increased core size cannot generally be made to operate only in a single mode, because the numerical aperture (NA) required for single-mode operation at large core sizes is lower than can be reliably achieved. Specifically, in existing fibers, the limited fine control of the core index increment restricts the NA to levels above 0.05. Another problem with the low-NA fibers is that they are sensitive to bending, and this fact complicates packaging. Different guiding-core structures and shapes to spread the mode size have been studied, including the addition of extra refractive index walls between the circular core and cladding. Coiling a fiber to strip out higher-order modes has been proposed and successfully used to select the lowest-order mode for LMA fiber lasers. Such efforts have resulted in the ability to raise the core diameter up to 30-50 μm. For core sizes larger than that, straight rod-like fibers have been shown to be useful for a high quality output beam.
The highest power reported for a nearly single-transverse mode fiber laser is presently about 3 kW. This power level approaches the practical limits estimated to be as high as 5-10 kilowatts for the CW regime. In the q-switched regime with nsec pulse durations, the state of the art is a few mJ, or peak powers of a few MW. Weapons-class applications, however, typically set 100 kW as a threshold for the laser system to become practically significant, and typical ladar sensor applications require peak powers of 10 MW or more. Few solutions have been proposed that can scale the output power of individual fiber lasers to this level. Most known proposed solutions involve combining parallel single-mode or LMA fiber channels. In principle, with this approach, the output power increases in proportion to the number of channels. However, maintaining a high beam quality and achieving a high combining efficiency (which means that the total combined power is equal to the sum of the powers of the individual fibers) have proven to be a challenge when addressing high power lasers.
In spite of extended efforts, fiber combining schemes have never reached the power levels achieved by direct LMA fiber sources. Nonlinear optical, thermal, and gain-saturation affects have so far limited the output power of fiber-combining schemes to a level that is nearly an order of magnitude below that possible with LMA designs. This suggests that fiber combining methods may not be capable of meeting the requirements of the intended applications.
An analysis of the present state of the art suggests that existing methods of raising fiber-laser output powers (either singly or combination) above 10 to 20 kilowatts are not capable of also delivering a high output beam quality.
Hence, a need exists in the art for an improved system or method for generating or amplifying higher output power laser beams than can be achieved with prior approaches, while maintaining high output beam quality.